Light amplifier

ABSTRACT

A light amplifier includes first and second light amplifying parts, and a first control part. The first light amplifying part has a first number of elements or ions performing a light amplifying operation and is excited by a first exciting light which is excessive with respect to the first number of elements or ions. The first light amplifying part amplifies a signal light applied hereto. The second light amplifying part has a second number of elements or ions performing a light amplifying operation and is excited by a second exciting light having a constant intensity. The second number is than the first number. The second light amplifying part amplifies an amplified signal light output from the first light amplifying part. The first control part performs a control operation so that an intensity of the amplified signal light output from the first light amplifying part is constant.

This is a division of application Ser. No. 08/530,875, filed Sep. 20,1995, now U.S. Pat. No. 5,664,131.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to light amplifiers, and moreparticularly to a light amplifier suitable for a situation in which alight amplifying operation is performed in a transmission path.

In optical communications in which signals are wavelength-multiplexed,it is required that the gain of a light amplifier is constant withrespect to the wavelengths of signal lights.

2. Description of the Related Art

A light output control has been reduced to practical use in which alight amplifier regulates the light output to a constant level. However,the dependence of the gain of the light amplifier with respect to thesignal wavelengths is still in the researching stage.

Several control methods for controlling the wavelength-dependence of thelight amplifier gain have been proposed. For example, the composition ofan amplifying medium is varied, or amplifying media having differentcompositions are combined together. Another proposed method is to use afilter having the function of compensating for the wavelength-dependenceof the amplifying medium.

However, the known control methods cannot remove the dependence of thewavelength-dependence of the light amplifier with respect to theintensity of an incident light. In this regard, a light amplifierutilizing a rare-earth-doped glass has been proposed. Another lightamplifier has also been proposed and marketed only for use in research,in which fluoride glass is used as host glass in order to reduce thewavelength-dependence of the cross section for emission and absorption.However, fluoride glass does not have a good reliability in the waterresistance of fluoride glass, and has a difficulty in splicing forconnections because the melting point thereof is quite different fromthat of normal glass. Further, there is a problem in which fluorideglass itself has a good reliability. For the above reasons, use offluoride glass is not suitable for applications to main routes ofcommercial communications services.

The wavelength-dependence of the light amplifier gain has aninput-light-intensity-dependence as shown in FIG. 1, which showscharacteristics of an erbium-doped fiber amplifier. More particularly,FIG. 1 shows gain differences with respect to the maximum gain whensweeping the probe light between 1550 nm and 1554 nm. In the prior art,there is no consideration of the input-light-intensity-dependence of thelight amplifier gain.

Normally, in the see-bottom optical filters, optical repeaters areintermittently provided in the optical fiber cable at given intervals.It is possible to equally arrange each of the optical repeaters so as tohave a light amplifying characteristic compensating for theinput-light-intensity-dependence, so that the wavelength-dependence ofthe light amplifier gain can be optimized. On the other hand, in theground light communications systems, light repeaters are not provided atgiven intervals. Hence, in order to optimize the light repeaters, it isrequired that each of the light repeaters be optimized taking into arespective installed location. This is troublesome and reduces theadvantages of use of the light amplifiers.

SUMMARY OF THE INVENTION

It is a general object of the present invention to provide a lightamplifier in which the above disadvantages are eliminated.

A more specific object of the present invention is to provide a lightamplifier in which the input-light-intensity-dependence thereof issuppressed and a constant wavelength dependence of the amplifier gainwith respect to a wide intensity of the input light.

The above objects of the present invention are achieved by a lightamplifier comprising: a first light amplifying part which has a firstnumber of elements or ions performing a light amplifying operation andwhich is excited by a first exciting light which is excessive withrespect to the first number of elements or ions, the first lightamplifying part amplifying a signal light applied hereto; a second lightamplifying part which has a second number of elements or ions performinga light amplifying operation and which is excited by a second excitinglight having a constant intensity, the second number being than thefirst number, the second light amplifying part amplifying an amplifiedsignal light output from the first light amplifying part; and a firstcontrol part which performs a control operation so that an intensity ofthe amplified signal light output from the first light amplifying partis constant.

The above first control part may control an intensity of the firstexciting light so that the intensity of the amplified signal lightoutput from the light amplifying part is constant.

The first control part may attenuate the signal light which is input tothe first light amplifier so that the intensity of the amplified signallight output from the light amplifying part is constant.

The above objects of the present invention are also achieved by a lightamplifier comprising: a first light amplifying part which is excited bya first exciting light and amplifies a signal light applied hereto; aninverted population generating part which is supplied with a secondexciting light having a wavelength different from that of the firstexciting light and which generates an inverted population of a level ofelements or ions related to amplifying based on the second excitinglight and another level not related to amplifying; a measuring partwhich measures an amplified spontaneous emission caused by the invertedpopulation; a first control part which controls a control operation sothat an intensity of an amplified signal light output from the firstlight amplifying part is constant; and a second control part whichcontrols the second exciting light so that a peak wavelength or a givenwavelength of the amplified spontaneous emission is constant based on aresult of a measurement by the measuring part.

The above measuring part may measure the amplified spontaneous emissiontraveling in a direction identical to that in which the signal lighttravels.

The above measuring part may measure the amplified spontaneous emissiontraveling in a reverse direction to a direction in which the signallight travels.

The above measuring part may measure a peak wavelength of the amplifiedspontaneous emission. The above measuring part may measure a givenwavelength of the amplified spontaneous emission.

The above objects of the present invention are also achieved by a lightamplifier comprising: a first light amplifying part which is excited bya first exciting light and amplifies a signal light applied hereto; aninverted population generating part which is supplied with a secondexciting light having a wavelength different from that of the firstexciting light and which generates an inverted population of a level ofelements or ions related to amplifying based on the second excitinglight and another level not related to amplifying; a second lightamplifying part which is supplied with the first exciting light and asecond exciting light and which has a loop in which an output light ofthe loop is input hereto as an input light, the second light amplifyingpart having an attenuator; an attenuation control part which controls anamount of attenuation of the attenuator on the basis of the signal lightinput to the first light amplifying part; a first control part whichcontrols an intensity of the second exciting light on the basis of anamplified spontaneous emission output from the attenuator; and a secondcontrol part which performs a control operation so that an intensity ofthe signal light output from the first light amplifying part isconstant.

The above objects of the present invention are also achieved by a lightamplifier comprising: a first light amplifying part which is excited bya first exciting light and amplifies a signal light applied hereto; aninverted population generating part which is supplied with a secondexciting light and generates an inverted population of elements or ionsperforming a light amplifying operation; and a first control part whichobtains a gain of the signal light by the first light amplifying partand a loss of the second exciting light and which controls an intensityof the first exciting light on the basis of the gain and the loss.

The above objects of the present invention are also achieved by a lightamplifier comprising: a first light amplifying part which has a variableamplifying medium and is excited by a first exciting light, the firstlight amplifying part amplifying a signal light applied hereto; a firstcontrol part which controls a length of the variable amplifying mediumof the first light amplifying part on the basis of an intensity of thesignal light applied to the first light amplifying part; and a secondcontrol part which controls an intensity of the first exciting light onthe basis of an amplified light signal output from the first lightamplifying part.

The above objects of the present invention are also achieved by a lightamplifier comprising: a first light amplifying part which is excited bya first exciting light and amplifies a signal light input hereto: afirst control part which controls an intensity of the first excitinglight so that a gain of the signal light by the first light amplifyingpart is constant; and a second control part which performs a controloperation so that an intensity of an amplified light signal output fromthe first light amplifying part is constant.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings, in which:

FIG. 1 is a graph of characteristics of an erbium-doped fiber amplifier;

FIG. 2 is a block diagram of a light amplifier according to a firstembodiment of the present invention;

FIG. 3 is a block diagram of a light amplifier according to a secondembodiment of the present invention;

FIG. 4 is a block diagram of a light amplifier according to a thirdembodiment of the present invention;

FIG. 5 is a graph showing a band structure of an erbium-doped fiber;

FIG. 6 is a block diagram of a light amplifier according to a fourthembodiment of the present invention;

FIG. 7 is a block diagram of a light amplifier according to a fifthembodiment of the present invention;

FIG. 8 is a block diagram of a light amplifier according to a sixthembodiment of the present invention;

FIG. 9 is a block diagram of a light amplifier according to a seventhembodiment of the present invention;

FIG. 10 is a block diagram of a light amplifier according to an eighthembodiment of the present invention;

FIG. 11 is a block diagram of a light amplifier according to a ninthembodiment of the present invention;

FIG. 12 is a block diagram of a light amplifier according to a tenthembodiment of the present invention;

FIG. 13 is a block diagram of a light amplifier according to an eleventhembodiment of the present invention;

FIG. 14 is a block diagram of a light amplifier according to a twelfthembodiment of the present invention;

FIG. 15 is a block diagram of a light amplifier according to athirteenth embodiment of the present invention;

FIG. 16 is a block diagram of a light amplifier according to afourteenth embodiment of the present invention; and

FIG. 17 is a block diagram of a light amplifier according to a fifteenthembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 is a block diagram of a light amplifier according to a firstembodiment of the present invention. A signal light of a wavelength inthe 1550 nm band or 1560 nm is applied to a light input terminal 10. Alight isolator 12 is formed with a Faraday element, and is provided inorder to prevent oscillation of the light amplifier. The signal lightpasses through the light isolator 12 and is applied to a wavelengthdistribution mirror (WDM) 14. The mirror 14, which is, for example, of afiber type, combines the signal light with a first exciting light of awavelength of 980 nm or 1480 nm which is supplied from a laser diode(LD) 16 and is different from that of the signal light. The signal lightwith the first exciting light combined is supplied to an erbium-dopedfiber (EDF) 18.

The erbium-doped fiber 18 has a small total amount of erbium regulatedby reducing the amount of doped erbium to a small level or reducing thefiber length L. The erbium-doped fiber 18 forms a first light amplifierin cooperation with the mirror 14 and the laser diode 16, and is in anexcessively excited state by the first exciting light from the laserdiode 16. The signal light is amplified through the erbium-doped fiber18 and is applied to a light intensity measuring unit 20. This unit 20is made up of a photocoupler 21 and a photodiode 22. Part of the signallight (for example, 1/11) separated therefrom by the photocoupler 21 issupplied to the photodiode 22, which detects the light intensity. Alight intensity signal output by the photodetector 22 is supplied to anautomatic level control circuit (ALC) 24, which controls the intensityof the exciting light emitted from the laser diode 16 on the basis ofthe light intensity signal. Hence, the intensity of the output light ofthe erbium-doped fiber 18 can be optimized to an optimal value of alight amplifier having a next-stage erbium-doped fiber 32. The abovephotodiode 22 and the automatic level control circuit 24 form a firstcontrol part.

Most of the signal light separated by the photocoupler 21 passes througha light isolator 26 using a Faraday element, and is applied to thewavelength distribution mirror (WDM) 28. The isolator 26 prevents anamplified spontaneous emission component which is produced in thenext-stage erbium-doped fiber 32 and is propagated in the reversedirection from entering the erbium-doped fiber 18 so that an oscillationtakes place. The light isolator 26 can be omitted.

The mirror 28 combines the signal light with a second exciting light ofa wavelength of 980 nm or 1480 nm supplied from a laser diode (LD) 30,and outputs a combined light to the erbium-doped fiber 32. A back powerdependent on the intensity of the exciting light emitted from the laserdiode 30 is applied to an automatic level control circuit (ALC) 34,which controls the laser diode 30 so that the laser diode 30 emits aconstant intensity of the exciting light. The erbium-doped fiber 32 hasa greater total amount of erbium by using a greater doping amount oferbium or increasing the fiber length L. The erbium-doped fiber 32 formsa second light amplifier in cooperation with the mirror 28 and the laserdiode 30. The intensity of the input light to the erbium-doped fiber 32is regulated to a fixed condition by supplying the optimized signallight from the erbium-doped fiber 18 and the second exciting light ofthe optimized intensity from the laser diode 30. The signal lightamplified by the erbium-doped fiber 32 passes through a light isolator36 for preventing oscillation, and is output via a light output terminal38.

The wavelength-dependence of the gain of the erbium-doped fiber can beexpressed as follows: ##EQU1## where G is the gain of the lightamplifier, σe(λ) is the absorption cross section of an amplifyingmedium, σa(λ) is the emission cross section thereof, N_(T) is the numberof amplifying substances per unit length of the amplifying medium, N₂ isthe number of amplifying substances per unit length which are excited atan upper level of a two-level system, N₁ is the number of amplifyingsubstances per unit length which a lower level corresponding to theground level, L is the length of the amplifying medium along thetraveling direction, and A is a constant.

It can be seen from equation (1) that the wavelength-dependence of thegain G depends on the values of N₂ and N₁ in the longitudinal directionof the amplifier, and the emission cross-section, while the magnitude ofthe wavelength-dependence depends on the number of elements having theamplifying function in the longitudinal direction of the amplifier. Thewavelength-dependence of the gain of the light amplifier depends on onlythe intensity of the exciting light and the intensity of the signallight if the amplifying medium and its density are determined.

From the above consideration, it is possible to remove theinput-light-intensity-dependence of the wavelength-dependence of theamplifier gain by:

1) operating the light amplifier at a constant gain;

2) controlling the total amount of elements which perform the amplifyingoperation based on the intensity of the input light; and/or

3) perform a control to regulate the amplifier input at a constantlevel.

The embodiment shown in FIG. 2 employs the erbium-doped fiber 18 havinga relatively smaller number of amplifying elements or amplifying ions,and the erbium-doped fiber 32 having a relatively large number ofamplifying elements or amplifying ions. It can be seen from equation (1)that the magnitude of a relative variation in the wavelength-dependenceof the gain is proportional to the number of elements or ions having theamplifying function, and the shape thereof is dependent on the value ofthe inverted population. With the above in mind, the intensity of theinput light to the second stage is subjected to the automatic levelcontrol at the first stage in which the wavelength-dependence of thegain is relatively small, as shown in FIG. 2. Hence, the intensity ofthe input light to the second stage having a large wavelength-dependenceof the gain can be regulated at the constant level, so that the lightamplifier having a small wavelength-dependence of the gain can beobtained.

In other words, in the first light amplifying part, a small total numberof elements or ions having the light amplifying operation is used andthe excited state is obtained by means of an excessive exciting light.Thus, the gain can be approximately constant and theinput-light-intensity-dependence of the wavelength-dependence of thegain can be reduced. Thereafter, the light intensity of the signal lightoutput from the second light amplifier part is made constant, so thatthe input-light-intensity-dependence of the wavelength-dependence of thegain can be suppressed. Further, the light intensity of the excitinglight can be controlled at the first control part, so that the intensityof the output signal light from the first light amplifying part can beregulated at the constant level with the gain controlled to theapproximately constant level.

FIG. 3 is a block diagram of a light amplifier according to a secondembodiment of the present invention. In FIG. 3, parts that are the sameas those shown in FIG. 2 are given the same reference numbers. Thesignal light of a wavelength in the 1550 nm band is supplied to anattenuator 40 through the light isolator 12. The attenuator 40 has avariable attenuation amount, which can be controlled by the automaticlevel control circuit 24. For example, the attenuator 40 includes alight switch which varies the attenuation amount by utilizing themagneto-optic effect or the acoustic-optic effect.

The signal light output from the attenuator 40 is supplied to the lightintensity measuring unit 20, which is made up of the photocoupler 21 andthe photodiode 21, as has been described previously. Part of the signallight separated by the photocoupler 21 is applied to the photodiode 22,which detects the intensity of the signal light. The light intensitysignal is supplied to the automatic level control circuit 24, whichcontrols the amount of attenuation of the light switch 40 on the basisof the above light intensity signal, so that the intensity of the signallight supplied to the light amplifier of the next stage becomes theoptimal value.

The mirror 28 combines the signal light with the second exciting lightof a wavelength of 980 nm or 1480 nm supplied from the laser diode (LD)30, and outputs the combined light to the erbium-doped fiber 32. Theback power dependent on the intensity of the exciting light emitted fromthe laser diode 30 is applied to the automatic level control circuit(ALC) 34, which controls the laser diode 30 so that the laser diode 30emits a constant intensity of the exciting light. The erbium-doped fiber32 has a greater total amount of erbium by using a greater doping amountof erbium or increasing the fiber length L. The erbium-doped fiber 32forms the second light amplifier in cooperation with the mirror 28 andthe laser diode 30. The intensity of the input light to the erbium-dopedfiber 32 is regulated to a fixed condition by supplying the signal lightof the optimized input light intensity by means of the light switch 40and the second exciting light of the optimized intensity from the laserdiode 30. The signal light amplified by the erbium-doped fiber 32 passesthrough the light isolator 36 for preventing oscillation, and is outputvia the light output terminal 38.

As described above, the input light to the erbium-doped fiber 32 iscontrolled to the constant level in the state in which the lightamplifier is constantly operated. Hence, the light amplifier can beoperated at the operating point at which the wavelength-dependence ofthe gain of the erbium-doped fiber 32 is optimized. In this way, theinput-light-intensity dependence of the wavelength-dependence of thegain can be removed.

FIG. 4 is a block diagram of a light amplifier according to a thirdembodiment of the present invention. In FIG. 4, parts that are the sameas those shown in the previously described figures are given the samereference numbers. The signal light having a wavelength falling in the1550 nm is applied to the light input terminal 10, and is supplied tothe mirror 14 through the light isolator 12 used to prevent oscillation.The mirror 14 is, for example, of a fiber type which combines the signallight with the first exciting light of a wavelength of 980 nm or 1480 nmwhich is supplied from the laser diode (LD) 16 and is different fromthat of the signal light. The signal light with the first exciting lightcombined is supplied to an erbium-doped fiber (EDF) 42.

The erbium-doped fiber 42 forms a third light amplifier part incooperation with the mirror 14 and the laser diode 16. The signal lightamplified in the third amplifier part is supplied to a light measuringpart 44. The erbium-doped fiber 42 is supplied to an exciting lighthaving a wavelength of 810 nm from a laser diode 58 (which will bedescribed later) through a wavelength distribution mirror 56. The aboveexciting light controls the hole burning or excited state absorption(ESA) of the erbium-doped fiber 42.

The light measuring part 44, which functions as an ASE (AmplifiedSpontaneous Emission) measuring part, is made up of a photocoupler 45, astar coupler 46, light band-pass fibers 48a through 48d, photodiodes 47,49a-49d, A/D converters 50a-50d, and a CPU 52. Part of the signal lightseparated by the photocoupler 45 (equal to, for example, 1/11) is madeto branch into five by the star coupler 46, so that the photodiode 47and the light band-pass fibers 48a-48d receive the respective parts ofthe signal light.

The photodiode 47 detects the light intensity, which outputs a lightintensity signal to the automatic level controller 24. The controller 24controls the intensity of the exciting light emitted from the laserdiode 16 so that the intensity of the output light of the erbium-dopedfiber 42 can be controlled to the constant level.

The light band-pass filters 48a-48d are optical filters having ahalf-value width of 1 nm, which allow the wavelengths 1540 nm, 1550 nm,1560 nm and 1570 nm to pass, respectively. Spontaneously emitted lightsfrom the respective light band-pass filters 48a-48d are input to thephotodiodes 47a-47d, which detect the respective light intensities Theintensities of the spontaneous emission lights in the respectivewavelength bands are digitized by the A/D converters 50a-50d, and arethen applied to the CPU 52.

The CPU 52 calculates the wavelength (peak wavelength) at which themaximum value can be obtained when fitting the light intensities of therespective bands to a three-order function by means of the least squaremethod. Then, the CPU 52 supplies a bias control circuit 54 (whichcorresponds to a third control part) with peak wavelength informationwhich indicates the value of the peak wavelength and the output lightintensity.

Most of the signal light separated by the photocoupler 45 is supplied tothe mirror 56. The mirror 56, which forms an inverted populationgenerating part, combines the signal light with a fourth exciting lighthaving a waveform of 850 nm supplied from the laser diode 58 in order tocause the ESA or hole burning. The back power dependent on the intensityof the exciting light emitted from the laser diode 58 is supplied to thebias control circuit 54. The circuit 54 controls, in the state in whichthe output light is subjected to the ALC based on the back power, theoutput of the laser diode 58 on the basis of the peak wavelengthinformation supplied from the CPU 52 so that the wavelength obtained atthe maximum value of the ASE is not changed. The signal light outputfrom the mirror 56 passes through a light isolator 60 used to preventoscillation, and is output via the light output terminal 38.

FIG. 5 schematically shows the band structure of a rare-earth-dopedfiber such as an erbium-doped fiber or a semiconductor/light amplifier.The elements or ions performing light amplifying are excited, by the 980nm exciting light, to a level at which induced emission can take place,and elements or ions at that level at which induced emission is possibleare further excited to a level at which the induced emission isimpossible (light-amplifying is impossible) due to the hole burning orESA caused by the exciting light of a wavelength of 850 nm. The symbolsN₁ and ₂ shown in FIG. 5 correspond to those in equation (1).

By applying the 850 nm light which causes the ESA or hole burning to theerbium-doped fiber 42, the value of N₂ at which the induced emissiontakes place can be controlled. By utilizing the above principle, it ispossible to change the gain of the amplifier in the state in which thevalue of the inverted population coefficient N₂ /(N₂ +N₁) can be madeconstant. The inverted population coefficient is a quantity thatcharacterizes the ASE spectrum shape, and therefore it is possible tooperate the light amplifier in the state in which the invertedpopulation coefficient is constant, by preventing a variation in thewavelength at which the maximum value is obtained, the maximum valuebeing obtained when comparing the intensities of three or morespontaneous emission lights having different wavelengths and performingfitting of the intensities by the least square method which uses afunction having an order of an integer equal to or less than the numberof waveform measuring points. It can be seen from equation (1) that thewavelength-dependence of the gain is determined only by the invertedpopulation coefficient. Hence, by using the above principle, it ispossible to realize the light amplifier in which thewavelength-dependence of the gain is not varied independently of theinput light. The third embodiment of the present invention utilizes themethod of controlling the total number of elements or ions performingthe amplifying operation based on the intensity of the input light.

FIG. 6 is a block diagram of a light amplifier according to a fourthembodiment of the present invention. In FIG. 6, parts that are the sameas those shown in FIG. 4 are given the same reference numbers. Thesignal light having a wavelength in the 1550 nm applied to the lightinput terminal 10 is supplied to the mirror 14 through the lightisolator 12. The mirror 14 combines the signal light with the excitinglight having a wavelength of 980 nm different from that of the signallight, the exciting light being supplied from the laser diode 16. Thesignal light with the exciting light combined is supplied to the lightmeasuring part 44.

The light measuring part 44 is made up of the photocoupler 45, the startcoupler 46, the light band-pass filters 48a-48d, photodiodes 49a-49d,A/D converters 50a-50d, and the CPU 52, as has been describedpreviously.

The light band-pass filters 48a-48d are optical filters having ahalf-value width of 1 nm, which allow the wavelengths 1540 nm, 1550 nm,1560 nm and 1570 nm to pass, respectively. Spontaneously emitted lightsfrom the respective light band-pass filters 48a-48d are input to thephotodiodes 47a-47d, which detect the respective light intensities. Theintensities of the spontaneous emission lights in the respectivewavelength bands are digitized by the A/D converters 50a-50d, and arethen applied to the CPU 52.

The CPU 52 calculates the wavelength (peak wavelength) at which themaximum value can be obtained when fitting the light intensities of therespective bands to a three-order function by means of the least squaremethod. Then, the CPU 52 supplies a bias control circuit 54 (whichcorresponds to a third control part) with peak wavelength informationwhich indicates the value of the peak wavelength and the output lightintensity.

Most of the signal light separated by the photocoupler 45 is supplied tothe erbium-doped fiber 42, which forms a light amplifier in cooperationwith the laser diode 16 and the mirror 14. The amplified signal light isapplied to the mirror 56. The erbium-doped fiber 42 is supplied with theexciting light having a wavelength of 850 nm from the laser diode 58 viathe mirror 56. The exciting light controls the hole burning or ESA ofthe erbium-doped fiber 42.

The mirror 56 combines the signal light with the exciting light having awavelength of, for example, 810 nm supplied from the laser diode 58 soas to cause the ESA or hole burning. The pack power dependent on theintensity of the exciting light emitted from the laser diode 58 issupplied to the bias control circuit 54. Further, the signal lightoutput from the mirror 56 is output to the light output terminal 38through the mirror 62 and the light isolator 60 used to preventoscillation.

Part of the signal light separated by the mirror 62 is supplied to aphotodiode 64, which detects the intensity of the received signal light.The automatic level control circuit 24 is supplied with the detectedlight intensity, and controls the intensity of the exciting lightemitted from the laser diode 16 so that the intensity of the outputlight from the erbium-doped fiber 42 is made constant.

The bias control circuit 54 controls the output of the laser diode 58 inthe state in which the output light is subjected to the automatic levelcontrol based on the back power so that the wavelength obtained at themaximum value of the ASE component is not changed.

Even in the fourth embodiment of the present invention, it is possibleto operate the light amplifier in the state in which the invertedpopulation coefficient is constant by comparing the intensities of threeor more spontaneous emission lights having different wavelengths andperforming fitting of the intensities by the least square method whichuses a function having an order of an integer equal to or less than thenumber of waveform measuring points. It can be seen from equation (1)that the wavelength-dependence of the gain is determined only by theinverted population coefficient. Hence, by using the above principle, itis possible to realize the light amplifier in which thewavelength-dependence of the gain is not varied independently of theinput light.

In the embodiment shown in FIG. 4 the ASE spectrum shape is observed bymonitoring the output light of the erbium-doped fiber 42, while in theembodiment shown in FIG. 6 the ASE spectrum traveling in the reversedirection with respect to the traveling direction of the signal light isobserved. Hence, there is a particular advantage in which the signallight little enters the light measuring part 44 and thus there is nolimitation regarding the wavelength of the light used as the signallight.

FIG. 7 is a block diagram of a light amplifier according to a fifthembodiment of the present invention. In FIG. 7, parts that are the sameas those shown in FIG. 6 are given the same reference numbers. Thesignal light having a wavelength in the 1550 nm applied to the lightinput terminal 10 is supplied to the mirror 14 through the lightisolator 12. The mirror 14 combines the signal light with the excitinglight having a wavelength of 980 nm different from that of the signallight and supplied from the laser diode 16. The combined light issupplied to the erbium-doped fiber 42.

The erbium-doped fiber 42 forms the light amplifier in cooperation withthe mirror 14 and the laser diode 16. The amplified signal light issupplied to a light measuring part 70.

The erbium-doped fiber 42 is supplied to an exciting light having awavelength of 850 nm from the laser diode via the mirror 56, so that thehole burning or ESA of the erbium-doped fiber 42 is controlled. Thelight measuring part 70 is made up of the photocoupler 45, aphotocoupler 72, light band-pass filters 73a and 73b, photodiodes 74aand 74b, and a comparator 75. Part of the signal light separated by thephotocoupler 45 is made to branch into two components by means of thephotocoupler 46. Then, the two light components are respectivelysupplied to the light band-pass filters 73a and 73b.

The light band-pass filters 73a and 73b are optical filters having ahalf-value width of 1 nm which respectively allow 1550 nm and 1560 nmlights to pass therethrough. The spontaneous emission lightsrespectively passing through the light band-pass filters 73a and 73b areapplied to the photodiodes 74a and 74b, which detect the intensities ofthe received lights. The intensities of the spontaneous emission lightsin the respective bands are compared by the comparator 75, whichsupplies the difference therebetween to a bias control circuit 76.

Most of the signal light separated by the photocoupler 24 is supplied tothe mirror 56, which combines the signal light with the exciting lightof a wavelength of 850 nm supplied from the laser diode 58 in order tocause the hole burning or ESA. The back power dependent on the intensityof the exciting light from the laser diode 58 is supplied to the biascontrol circuit 76. Further, the signal light output from the mirror 56passes through the mirror 62 and the light isolator 60 used to preventoscillation, and is output via the light output terminal 38.

Part of the signal light separated by the mirror 62 is supplied to thephotodiode 64, which detects the intensity of the signal light. The ALCcircuit 24 is notified of the detected light intensity, and controls theintensity of the exciting light emitted from the laser diode 16 so thatthe intensity of the output light of the erbium-doped fiber 42 isconstant.

The bias control circuit 76 controls the output of the laser diode 58 onthe basis of the comparison result obtained at the comparator 75 in thestate in which the output light is subjected to the automatic levelcontrol so that the shape of the ASE spectrum is not changed. Forexample, if the output values of the photodiodes 74a and 74b are thesame, a given ASE spectrum shape is obtained. The output of thephotodiode 74a is greater than that of the photodiode 74b, there is anexcessive number of elements or ions related to the light amplifying. Inthis case, the output of the laser diode 58 is increased in order toincrease the amount of ESA or hole burning and thus equalize the outputvalues of the photodiodes 74a and 74b.

That is, it is possible to operate the light amplifier in the state inwhich the inverted population coefficient is constant. Hence, it ispossible to vary the gain of the amplifier in the state in which thevalue of the inverted population coefficient N₂ /(N₂ +N₁) is constant.The wavelength-dependence of the gain is determined only by the invertedpopulation coefficient from equation (1), and it is thus possible torealize the light amplifier in which there is no variation in thewavelength-dependence of the gain independently of the input light. Thenumber of wavelength points to be monitored may be an arbitrary numberequal to or greater than 1 on the basis of the ASE shape.

FIG. 8 is a block diagram of a light amplifier according to a sixthembodiment of the present invention. In FIG. 8, parts that are the sameas those shown in FIG. 7 are given the same reference numbers. Thestructure shown in FIG. 8 differs from that shown in FIG. 7 in that thelight measuring part 70 is located at a stage in advance of theerbium-doped fiber 42. This is the same relation as that between thestructures shown in FIGS. 4 and 6.

Even according to the embodiment shown in FIG. 8, it is possible tooperate the light amplifier in the state in which the invertedpopulation coefficient is constant. Hence, it becomes possible tooperate the gain of the amplifier in the state in which the invertedpopulation coefficient N₂ /(N₂ +N₁) is constant. The waveform-dependenceof the gain is determined only by the inverted population coefficientfrom equation (1). Hence, by utilizing the above principle, it ispossible to realize the light amplifier in which thewavelength-dependence of the gain is not changed independently of theinput light. The number of wavelength points to be monitored may be anarbitrary number equal to or greater than 1 on the basis of the ASEshape.

In the embodiment shown in FIG. 7, the shape of the ASE spectrum isobserved by monitoring the output light of the erbium-doped fiber 42,while in the embodiment shown in FIG. 8 the ASE spectrum traveling inthe reverse direction with respect to the traveling direction of thesignal light is observed. Hence, there is a particular advantage inwhich the signal light little enters the light measuring part 44 andthus there is no limitation regarding the wavelength of the light usedas the signal light.

FIG. 9 is a block diagram of a light amplifier according to a seventhembodiment of the present invention. In FIG. 9, parts that are the sameas those shown in FIG. 2 are given the same reference numbers. Thesignal light having a wavelength in the 1550 nm is applied to the lightinput terminal 10, and is then applied to a coupler 80 via the lightisolator 12 used to prevent oscillation. The coupler 80 supplies aphotodetector 81 with part of the signal light. The photodetector 81detects the intensity of the received signal light, and notifies acontrol circuit of the detected light intensity.

Most of the signal light is supplied to the mirror 14 from the coupler80. The mirror 14 is supplied, through a coupler 83, with the excitinglight having a wavelength of, for example, 980 nm generated by the laserdiode 16. The mirror 14 combines the signal light with the excitinglight, and supplies the combined wave to the erbium-doped fiber 42,which has a relatively short length and amplifies the received signallight. The erbium-doped fiber 42 is supplied, through a coupler 84 andthe mirror 56, with the exciting light having a wavelength of, forexample, 850 nm generated by the laser diode 58, so that the holeburning or ESA takes place.

Part of the exciting light having a wavelength of 980 nm separated bythe coupler 83 is combined with a loop light by means of a wavelengthdistribution mirror 86. The combined light is supplied to anerbium-doped fiber 88, which forms the fourth light amplifier. Part ofthe 850 nm exciting light separated by the coupler 84 is combined withthe loop light by means of a wavelength distribution mirror 90. Thecombined light is supplied to the erbium-doped fiber 88. An ASE cyclicsystem is formed by the mirror 86, a light isolator 91, an attenuator(ATT) 92, a coupler 93, the mirror 90, and the erbium-doped fiber 88. Inthe ASE cyclic system, the level of a wavelength having a large gain isincreased so that a peak is formed.

When the outputs of the laser diodes 16 and 58 are at respectiveconstant levels, the loop gain of the ASE cyclic system is varied bychanging the amount of attenuation at the attenuator 92 by means of thecontrol circuit 82 in accordance with the intensity of the input signallight. Due to the input-light-intensity-dependence of thewavelength-dependence of the gain, the wavelength of the peak in the ASEcyclic system is varied.

Now, the pre-adjustment is performed by varying the intensity of theinput signal light so that the wavelength of the peak of the gain of theerbium-doped fibers is not changed. This is done by adjusting the outputvalue of the laser diode 16 and the output value of the laser diode 58.The amount of attenuation of the attenuator 92 is measured so that thepeak wavelength of the ASE cyclic system is not changed when adjustingthe laser diodes 16 and 58. The information thus obtained (the amount ofattenuation of the attenuator 92 and the intensity of the input signallight) is stored in the control circuit 82.

Part of the loop light separated by the coupler 93 of the ASE cyclicsystem is supplied to a photodetector 96 via a band-pass filter 94 whichallows the peak wavelength of the ASE cyclic system to passtherethrough. The photodetector 96 detects the intensity of the peakwavelength, and notifies the control circuit 98 (which functions as thefourth control part) of the detected light intensity. The controlcircuit 98 controls the laser diode 58 so that the maximum lightintensity can be obtained at the peak wavelength.

The output light of the mirror 56 passes through the coupler 62 and thelight isolator 60, and is output via the terminal 38. Part of the outputlight extracted by the coupler 62 is supplied to the photodetector 64,which detects the intensity of the output light. The automatic levelcontrol circuit 24, which functions as the fourth control part, controlsthe laser diode 16 on the basis of the detected light intensity so thatthe intensity of the output light is constant.

That is, the intensity of the exciting lights respectively emitted fromthe laser diodes 16 and 58 are controlled so that the ASE peakwavelength is the same as that obtained in the adjustment by varying theamount of attenuation of the attenuator 92 on the basis of the intensityof the input light. Hence, the wavelength-dependence of the gain of thelight amplifier becomes independent of the intensity of the input light.

The laser diodes 16 and 58 are respectively controlled by the automaticlevel control circuit 24 and the control circuit 98, rather than acontrol process in which the control amounts related to the values ofthe intensity of the input light. This is intended to prevent a changein the performance of the light amplifier due to age deterioration ofthe performances of the laser diodes 16 and 58.

According to the seventh embodiment of the present invention, it ispossible to make the light amplifier operated in the state in which theinverted population of the third light amplifying part is constant andto thereby suppress a variation in the wavelength-dependence of the gaindue to the intensity of the input light. This is achieved by forming theASE cyclic system including the fourth amplifier which operates with thelight intensity proportional to the output light of the erbium-dopedfiber 42 serving as the third light amplifying part, and controlling theintensity of the fourth exciting light so that the peak wavelength ofthe ASE cyclic system is not varied in the state in which the thirdamplifier has the optimal wavelength-dependence of the gain.

FIG. 10 is a block diagram of a light amplifier according to an eightembodiment of the present invention. In FIG. 10, parts that are the sameas those shown in FIG. 2 are given the same reference numbers. In FIG.10, the signal light which is applied to the light input terminal 10 andhas a wavelength falling in, for example, 1550 nm passes through thelight isolator 12, and is supplied to a wavelength distribution mirror104 via couplers 100 and 102. The mirror 140 combines the signal lightwith the 810 nm exciting light generated by the laser diode 58, and thecombined light is supplied to the erbium-doped fiber 42.

The mirror 106 is supplied, via a coupler 110, with the 980 nm excitinglight generated by the laser diode 16, and combines the signal lightwith the supplied exciting light. In the erbium-doped fiber 42, thesignal light is amplified by the 980 nm exciting light. The hole burningor ESA is caused by the 850 nm exciting light. The signal lightamplified by the erbium-doped fiber 42 is output via the mirror 106, andpasses through a coupler 108 and the light isolator 60 used to preventoscillation. Then, the signal light is output via the light outputterminal 38.

Part of the 980 nm exciting light is separated by the coupler 100, and aphotodetector 112 detects the intensity of the exciting light. Part ofthe input signal light is separated by the coupler 102, and theintensity thereof is detected by a photodetector 114.

Part of the output signal light is separated by the coupler 108, and theintensity thereof is detected by a photodetector 116. An automatic levelcontrol circuit 118 controls the laser diode 16 so that the intensity ofthe output signal light is constant. Part of the 980 nm exciting lightseparated by the coupler 110 is supplied to the photodetector 120, whichdetects the intensity of the exciting light.

A loss computation part 122 computes a loss of the 980 nm exciting lightfrom the intensities of the output lights of photodetectors 120 and 112.A gain computation part 124 computes the gain of the signal light fromthe intensities of the lights detected by the photodetectors 116 and114. A control circuit 126 obtains the ratio of the logarithm of theloss of the exciting light to the logarithm of the gain of the signallight, and thereby controls the laser diode 58 so that the invertedpopulation coefficient is constant and the wavelength-dependence of thegain is not varied independently of the intensity of the input light. Asixth control part is formed by the loss computation part 122, the gaincomputation part 124 and the control circuit 126.

The loss "Loss" of the exciting light (980 nm) can be expressed byequation (2): ##EQU2## The gain G and the loss Loss can also beexpressed as follows: ##EQU3## where N and T are respectively theaverages of N₂ and N_(T) in the longitudinal direction, t is theinverted population coefficient, and σap is the absorption cross-sectionof the amplifying medium with respect to pump light.

From the above, the following equation stands:

    log(G)/log(Loss)= (σa+σe)/σap!× t/(1-t)!

    -(σa/σap)× 1/(1-t)!                      (5)

In equation (5), σa and σe are constants and the inverted populationcoefficient t is made constant by setting Log(G)/Log(Loss).

In the eighth embodiment of the present invention, the intensity of theexciting light is controlled by referring to the gain of the signallight and the loss of the exciting light. Hence, it is possible toperform the light amplifying operation in the state in which theinverted population coefficient is constant and suppress theinput-light-intensity-dependence of the wavelength-dependence of thegain.

FIG. 11 is a block diagram of a light amplifier according to a ninthembodiment of the present invention. In FIG. 11, parts that are the sameas those shown in FIG. 2 are given the same reference numbers. In FIG.11, the signal light having a wavelength falling in the 1550 nm isapplied to the light input terminal 10, and is supplied to a coupler 127via the light isolator 12. Part of the signal light separated by thecoupler 127 is supplied to a photodetector 128, which detects theintensity of the input signal light. The signal light mostly passingthrough the coupler 127 is supplied to the wavelength distributionmirror 14, which combines the signal light with the 980 nm excitinglight emitted from the laser diode 16. The combined light is supplied toan erbium-doped fiber 130.

The erbium-doped fibers 130, 132, 134, 136 and 138 are connected inseries via switches 131, 133, 135 and 137. Each of the switches 131,133, 135 and 137 connects the erbium-doped fiber located at the frontstage thereof to either the erbium-doped fiber located at the rear stageor a switch 140. The above switching operations of the switches 131,133, 135 and 137 are controlled in accordance with control signalsoutput from a control circuit 142. The ratio of the lengths of theerbium-doped fibers 130, 132, 134, 136 and 138 is equal to, for example,5:1:1:1:1. The length of the erbium-doped fiber contributing toamplifying of the light can be varied by the switching operations of theswitches 131, 133, 135 and 137. These erbium-doped fibers and switchesform a fifth light amplifying part.

The signal light amplified by any of the erbium-doped fibers 130, 132,134, 136 and 138 is output via the switch 140 controlled by the controlcircuit 142, and is then supplied to a coupler 144. The coupler 144outputs most of the signal light to the light output terminal 38 via thelight isolator 60 used to prevent oscillation. Part of the signal lightseparated by the coupler 144 is supplied to a photodetector 146, whichdetects the intensity of the output signal light.

A/D converters 150 and 152 respectively digitize the values of theintensities of the input signal light and output signal lightrespectively detected by the photodetectors 128 and 146, and outputdigitized values to a CPU 154. The CPU 154 sends an instruction based onthe intensity of input signal light to the control circuit 142, whichcontrols the switches 131, 133, 135 and 137 to vary the length of theerbium-doped fiber for light amplification. As the intensity of theinput signal light becomes greater, the length of the erbium-doped fiberis made shorter. Thereafter, a control circuit 156, which functions asan eighth control part, controls the laser diode 16 so that theintensity of the output signal light is constant.

In the ninth embodiment of the present invention, it is possible toregulate the total amount of elements or ions really amplifying thelight at an approximately constant level independently of the intensityof the input signal light and to hence suppress theinput-light-intensity-dependence of the waveform-dependence of the gain.

FIG. 12 is a block diagram of a light amplifier according to a tenthembodiment of the present invention. In FIG. 12, parts that are the sameas those shown in FIG. 4 or FIG. 11 are given the same referencenumbers. In FIG. 12, the signal light having a wavelength in the 1550 nmband is applied to the light input terminal 10, and is supplied to acoupler 127 via the light isolator 12. Part of the signal lightseparated by the coupler 127 is supplied to a photodetector 128, whichdetects the intensity of the input signal light. Most of the signallight passing through the coupler 127 is supplied to the wavelengthdistribution mirror 14, which combines the received signal light withthe 980 nm exciting light generated by the laser diode 16. The combinedlight is supplied to the erbium-doped fiber 42.

A band-pass filter 159 eliminates unwanted components from the signallight amplified by the erbium-doped fiber 42, the output signal of thefilter 159 being supplied to the coupler 144. The coupler 144 suppliesmost of the signal light to an attenuator 160 through the light isolator60 used to prevent oscillation. Part of the signal light separated bythe light coupler 144 is supplied to the photodetector 146, whichdetects the intensity of the output signal light.

An automatic gain control circuit 158, which functions as a ninthcontrol part, controls the laser diode 16 so that the ratio of theintensity of the output signal light detected by the photodetector 146to that of the input signal light detected by the photodetector 128 isconstant, that is, the gain is constant.

The output signal light attenuated by the attenuator 160 is supplied toa coupler 162, which outputs most of the received output signal light tothe light output terminal 38. Part of the output signal light separatedby the coupler 162 is supplied to a photodetector 164, which detects theintensity of the output signal light. An automatic level control circuit166, which functions as a tenth control part, controls the attenuator160 so that the intensity of the output signal light is constant. Theuse of the attenuator 160 is intended to canceling a variation of theintensity of the signal light amplified by the erbium-doped fiber due toa variation in the intensity of the input signal light while the gain ofthe erbium-doped fiber 42 is constant.

FIG. 13 is a block diagram of a light amplifier according to an eleventhembodiment of the present invention in which the configuration shown inFIG. 2 is modified so that the front stage between the light inputterminal 10 and the light isolator 26 shown in FIG. 2 is replaced by theconfiguration shown in FIG. 12. In FIG. 13, the 1550 nm signal light isapplied to the light input terminal 10, and is supplied to the coupler127 via the light isolator 12. Part of the signal light separated by thecoupler 127 is supplied to the photodetector 128, which detects theintensity of the input signal light. The most of the signal lightpassing through the coupler 127 is supplied to the wavelengthdistribution mirror 14, which combines the signal light with the 980 nmexciting light generated by the laser diode 16. The combined light issupplied to the erbium-doped fiber 42. The erbium-doped fiber 42 has arelatively short length and is excessively excited by the exciting lightfrom the laser diode 16.

The band-pass filter 159 eliminates unwanted wavelength components fromthe signal light amplified by the erbium-doped fiber 42, the outputsignal thereof being applied to the coupler 144. The coupler 144supplies most of the received signal light to the attenuator 160 via thelight isolator 60. Part of the signal light separated by the lightcoupler 144 is supplied to the photodetector 146, which detects theintensity of the output signal light.

The automatic gain control circuit 158 controls the laser diode 16 sothat the ratio of the intensity of the output signal light detected bythe photodetector 146 to that of the input signal light detected by thephotodetector 128 is constant, that is, the gain is constant.

The output signal light attenuated by the attenuator 160 is supplied tothe coupler 162, which outputs most of the received output signal lightto the light output terminal 38. Part of the output signal lightseparated by the coupler 162 is supplied to the photodetector 164, whichdetects the intensity of the output signal light. The automatic levelcontrol circuit 166 controls the attenuator 160 so that the intensity ofthe output signal light is constant. Hence, the intensity of the outputlight of the erbium-doped fiber 42 is optimized so that it becomes equalto the optimal value for the next-stage light amplifier including theerbium-doped fiber 32.

The most of the signal light separated by the light coupler 162 issupplied to the mirror 28, which combines the signal light with the 980nm or 1480 nm exciting light supplied from the laser diode 16. Thecombined light is supplied to the erbium-doped fiber 32. The back powerbased on the intensity of the exciting light from the laser diode 16 issupplied to the automatic level control circuit 34. The circuit 34controls the laser diode 30 so that the intensity of the exciting lightemitted from the laser diode 30 is constant. The erbium-doped fiber 32forms a light amplifier in cooperation with the mirror 28 and the laserdiode 30, and has a larger amount of erbium or a longer length than theerbium-doped fiber 42. The erbium-doped fiber 32 is supplied with thesignal light having the intensity optimized by the erbium-doped fiber 42and the exciting light having the optimized intensity from the laserdiode 30. That is, the intensity of the input light to the erbium-dopedfiber 32 is made constant. The signal light amplified by theerbium-doped fiber 32 passes through the light isolator 36 used toprevent oscillation, and is output to the light output terminal 38.

In the eleventh embodiment of the present invention, the output light ofthe front-stage light amplifier is amplified by the rear-stage lightamplifier having a constant gain, so that theinput-light-intensity-dependence of the wavelength-dependence of thegain can be suppressed and a large signal gain can be obtained.

FIG. 14 is a block diagram of a light amplifier according to a twelfthembodiment of the present invention in which the configuration shown inFIG. 2 is modified so that the front stage between the light inputterminal 10 and the light isolator 26 shown in FIG. 2 is replaced by theconfiguration shown in FIG. 4. The input signal light in the 1550 nmband applied to the light input terminal 10 passes through the lightisolator 12 used to prevent oscillation, and is supplied to thewavelength distribution mirror 14. The mirror 14 is of a fiber type,which combines the signal light with the exciting light having awavelength of 980 nm different from that of the signal light andsupplied from the laser diode 16. The signal with the exciting lightcombined is supplied to the erbium-doped fiber 42 having a relativelyshort length. The erbium-doped fiber 42 is excessively excited by theexciting light from the laser diode 16.

The erbium-doped fiber 42 forms the light amplifier in cooperation withthe mirror 14 and the laser diode 16. The signal light amplified by theerbium-doped fiber 42 is supplied to the light measuring part 44. Theerbium-doped fiber 42 is supplied with the 850 nm exciting light fromthe laser diode 58 via the mirror 56 whereby the hole burning or the ESAof the erbium-doped fiber 42 is controlled.

The light measuring part 44 is made up of the photocoupler 45, the starcoupler 46, the light band-pass filters 48a-48d, the photodiodes 47,49a-49d, the A/D converters 50a-50d and the CPU 52.

Part of the signal light separated by the photocoupler 45 (equal to, forexample, 1/11) is made to branch into five by the star coupler 46, sothat the photodiode 47 and the light band-pass fibers 48a-48d receivethe respective parts of the signal light.

The photodiode 47 detects the light intensity, which outputs a lightintensity signal to the automatic level controller 24. The controller 24controls the intensity of the exciting light emitted from the laserdiode 16 so that the intensity of the output light of the erbium-dopedfiber 42 can be controlled to the constant level.

The light band-pass filters 48a-48d are optical filters having ahalf-value width of 1 nm, which allow the wavelengths 1540 nm, 1550 nm,1560 nm and 1570 nm to pass, respectively. Spontaneously emitted lightsfrom the respective light band-pass filters 48a-48d are input to thephotodiodes 47a-47d, which detect the respective light intensities. Theintensities of the spontaneous emission lights in the respectivewavelength bands are digitized by the A/D converters 50a-50d, and arethen applied to the CPU 52.

The CPU 52 calculates the wavelength (peak wavelength) at which themaximum value can be obtained when fitting the light intensities of therespective bands to a three-order function by means of the least squaremethod. Then, the CPU 52 supplies the bias control circuit 54 with peakwavelength information which indicates the value of the peak wavelengthand the output light intensity.

Most of the signal light separated by the photocoupler 45 is supplied tothe mirror 56. The mirror 56 combines the signal light with a fourthexciting light having a waveform of 850 nm supplied from the laser diode58 in order to cause the ESA or hole burning. The back power dependenton the intensity of the exciting light emitted from the laser diode 58is supplied to the bias control circuit 54. The circuit 54 controls, inthe state in which the output light is subjected to the ALC based on theback power, the output of the laser diode 58 on the basis of the peakwavelength information supplied from the CPU 52 so that the wavelengthobtained at the maximum value of the ASE is not changed.

By applying the 850 nm light which causes the ESA or hole burning to theerbium-doped fiber 42, the value of N₂ at which the induced emissiontakes place can be controlled. By utilizing the above principle, it ispossible to change the gain of the amplifier in the state in which thevalue of the inverted population coefficient N₂ /(N₂ +N₁) can be madeconstant. The inverted population coefficient is a quantity thatcharacterizes the ASE spectrum shape, and therefore it is possible tooperate the light amplifier in the state in which the invertedpopulation coefficient is constant, by preventing a variation in thewavelength at which the maximum value is obtained, the maximum valuebeing obtained when comparing the intensities of three or morespontaneous emission lights having different wavelengths and performingfitting of the intensities by the least square method which uses afunction having an order of an integer equal to or less than the numberof waveform measuring points. It can be seen from equation (1) that thewavelength-dependence of the gain is determined only by the invertedpopulation coefficient. Hence, by using the above principle, it ispossible to realize the light amplifier in which thewavelength-dependence of the gain is not varied independently of theinput light.

The intensity of the output light of the erbium-doped fiber 42 isoptimized so that it has the optimal value of the next-stage lightamplifier having the erbium-doped fiber 32. The signal light output bythe mirror 56 passes through the light isolator 60 used to preventoscillation and is supplied to the mirror 28. The mirror 28 combines thesignal light with the 980 nm or 1480 nm exciting light supplied from thelaser diode 16, the combined light being applied to the erbium-dopedfiber 32. The back power dependent on the intensity of the excitinglight from the laser diode 16 is supplied to the automatic level controlcircuit 34, which controls the laser diode 30 so that the intensity ofthe exciting light emitted from the laser diode 30 is constant.

The erbium-doped fiber 32 forms a light amplifier in cooperation withthe mirror 28 and the laser diode 30, and has a larger amount of erbiumor a longer length than the erbium-doped fiber 42. The erbium-dopedfiber 32 is supplied with the signal light having the intensityoptimized by the erbium-doped fiber 42 and the exciting light having theoptimized intensity from the laser diode 30. That is, the intensity ofthe input light to the erbium-doped fiber 32 is made constant. Thesignal light amplified by the erbium-doped fiber 32 passes through thelight isolator 36 used to prevent oscillation, and is output to thelight output terminal 38.

In the twelfth embodiment of the present invention, the output light ofthe front-stage light amplifier is amplified by the rear-stage lightamplifier having a constant gain, so that theinput-light-intensity-dependence of the wavelength-dependence of thegain can be suppressed and a large signal gain can be obtained.

FIG. 15 is a block diagram of a light amplifier according to athirteenth embodiment of the present invention in which theconfiguration shown in FIG. 2 is modified so that the front stagebetween the light input terminal 10 and the light isolator 26 shown inFIG. 2 is replaced by the configuration shown in FIG. 6. The inputsignal light in the 1550 nm band applied to the light input terminal 10passes through the light isolator 12 used to prevent oscillation, and issupplied to the wavelength distribution mirror 14. The mirror 14 is of afiber type, which combines the signal light with the exciting lighthaving a wavelength of 980 nm different from that of the signal lightand supplied from the laser diode 16. The signal with the exciting lightcombined is supplied to the light measuring part 44.

As has been described previously, the light measuring part 44 is made upof the photocoupler 45, the star coupler 46, the light band-pass fibers48a through 48d, the photodiodes 47, 49a-49d, A/D converters 50a-50d,and the CPU 52. Part of the signal light separated by the photocoupler45 (equal to, for example, 1/11) is made to branch into five by the starcoupler 46, so that the photodiode 47 and the light band-pass fibers48a-48d receive the respective parts of the signal light.

As has been described previously, the light band-pass filters 48a-48dare optical filters having a half-value width of 1 nm, which allow thewavelengths 1540 nm, 1550 nm, 1560 nm and 1570 nm to pass, respectively.Spontaneously emitted lights from the respective light band-pass filters48a-48d are input to the photodiodes 47a-47d, which detect therespective light intensities. The intensities of the spontaneousemission lights in the respective wavelength bands are digitized by theA/D converters 50a-50d, and are then applied to the CPU 52.

The CPU 52 calculates the wavelength (peak wavelength) at which themaximum value can be obtained when fitting the light intensities of therespective bands to a three-order function by means of the least squaremethod. Then, the CPU 52 supplies the bias control circuit 54 with peakwavelength information which indicates the value of the peak wavelengthand the output light intensity.

Most of the signal light separated by the light coupler 45 is suppliedto the erbium-doped fiber 42, which has a relatively short length, andforms the light amplifier in cooperation with the laser diode 16 and thewavelength distribution mirror 14. The signal light amplified by theabove light amplifier is supplied to the wavelength distribution mirror56. The erbium-doped fiber 42 is supplied with the 850 nm exciting lightemitted from the laser diode 58 through the mirror 56, whereby the holeburning or ESA of the erbium-doped fiber 42 is controlled.

The mirror 56 combines the signal light with the exciting light having awavelength of, for example, 810 nm supplied from the laser diode 58 soas to cause the ESA or hole burning. The pack power dependent on theintensity of the exciting light emitted from the laser diode 58 issupplied to the bias control circuit 54. Further, the signal lightoutput from the mirror 56 is output to the mirror 28 through the mirror62 and the light isolator 60 used to prevent oscillation.

Part of the signal light separated by the mirror 62 is supplied to thephotodiode 64, which detects the intensity of the received signal light.The automatic level control circuit 24 is supplied with the detectedlight intensity, and controls the intensity of the exciting lightemitted from the laser diode 16 so that the intensity of the outputlight from the erbium-doped fiber 42 is made constant.

The bias control circuit 54 controls the output of the laser diode 58 inthe state in which the output light is subjected to the automatic levelcontrol based on the back power so that the wavelength obtained at themaximum value of the ASE component is not changed.

It is possible to operate the light amplifier in the state in which theinverted population coefficient is constant by comparing the intensitiesof three or more spontaneous emission lights having differentwavelengths and performing fitting of the intensities by the leastsquare method which uses a function having an order of an integer equalto or less than the number of waveform measuring points. It can be seenfrom equation (1) that the wavelength-dependence of the gain isdetermined only by the inverted population coefficient. Hence, by usingthe above principle, it is possible to realize the light amplifier inwhich the wavelength-dependence of the gain is not varied independentlyof the input light. In this way, the intensity of the output light ofthe erbium-doped fiber 42 is optimized so that it has an optimal valueto the next-stage light amplifying part including the erbium-doped fiber32.

The mirror 28 combines the signal light with the 980 nm or 1480 nmexciting light supplied from the laser diode 16, the combined lightbeing applied to the erbium-doped fiber 32. The back power dependent onthe intensity of the exciting light from the laser diode 16 is suppliedto the automatic level control circuit 34, which controls the laserdiode 30 so that the intensity of the exciting light emitted from thelaser diode 30 is constant.

The erbium-doped fiber 32 forms a light amplifier in cooperation withthe mirror 28 and the laser diode 30, and has a larger amount of erbiumor a longer length than the erbium-doped fiber 42. The erbium-dopedfiber 32 is supplied with the signal light having the intensityoptimized by the erbium-doped fiber 42 and the exciting light having theoptimized intensity from the laser diode 30. That is, the intensity ofthe input light to the erbium-doped fiber 32 is made constant. Thesignal light amplified by the erbium-doped fiber 32 passes through thelight isolator 36 used to prevent oscillation, and is output to thelight output terminal 38.

In the thirteenth embodiment of the present invention, the output lightof the front-stage light amplifier is amplified by the rear-stage lightamplifier having a constant gain, so that theinput-light-intensity-dependence of the wavelength-dependence of thegain can be suppressed and a large signal gain can be obtained.

FIG. 16 is a block diagram of a light amplifier according to afourteenth embodiment of the present invention in which theconfiguration shown in FIG. 2 is modified so that the front stagebetween the light input terminal 10 and the light isolator 26 shown inFIG. 2 is replaced by the configuration shown in FIG. 9. The inputsignal light in the 1550 nm band applied to the light input terminal 10passes through the light isolator 12 used to prevent oscillation, and issupplied to the coupler 80. Part of the signal light is separated by thecoupler 80, and is then supplied to the photodetector 81, which detectsthe intensity of the signal light. The control circuit 82 is notified ofthe detected intensity of the signal light.

Most of the signal light is supplied to the mirror 14 from the coupler80. The mirror 14 is supplied, through the coupler 83, with the excitinglight having a wavelength of, for example, 980 nm generated by the laserdiode 16. The mirror 14 combines the signal light with the excitinglight, and supplies the combined wave to the erbium-doped fiber 42,which has a relatively short length. The erbium-doped fiber 42 amplifiesthe signal light. The erbium-doped fiber 42 is supplied, through thecoupler 84 and the mirror 56, with the exciting light having awavelength of, for example, 850 nm generated by the laser diode 58, sothat the hole burning or ESA takes place.

Part of the exciting light having a wavelength of 980 nm separated bythe coupler 83 is combined with the loop light by means of thewavelength distribution mirror 86. The combined light is supplied to theerbium-doped fiber 88, which forms the fourth light amplifier. Part ofthe 850 nm exciting light separated by the coupler 84 is combined withthe loop light by means of the wavelength distribution mirror 90. Thecombined light is supplied to the erbium-doped fiber 88. As has beendescribed previously, the ASE cyclic system is formed by the mirror 86,the light isolator 91, the attenuator (ATT) 92, the coupler 93, themirror 90, and the erbium-doped fiber 88. In the ASE cyclic system, thelevel of a wavelength having a large gain is increased so that a peak isformed.

When the outputs of the laser diodes 16 and 58 are at respectiveconstant levels, the loop gain of the ASE cyclic system is varied bychanging the amount of attenuation at the attenuator 92 by means of thecontrol circuit 82 in accordance with the intensity of the input signallight. Due to the input-light-intensity-dependence of thewavelength-dependence of the gain, the wavelength of the peak in the ASEcyclic system is varied.

Now, the pre-adjustment is performed by varying the intensity of theinput signal light so that the wavelength of the peak of the gain of theerbium-doped fibers is not changed. This is done by adjusting the outputvalue of the laser diode 16 and the output value of the laser diode 58.The amount of attenuation of the attenuator 92 is measured so that thepeak wavelength of the ASE cyclic system is not changed when adjustingthe laser diodes 16 and 58. The information thus obtained (the amount ofattenuation of the attenuator 92 and the intensity of the input signallight) is stored in the control circuit 82.

Part of the loop light separated by the coupler 93 of the ASE cyclicsystem is supplied to the photodetector 96 via the band-pass filter 94which allows the peak wavelength of the ASE cyclic system to passtherethrough. The photodetector 96 detects the intensity of the peakwavelength, and notifies the control circuit 98 (which functions as thefourth control part) of the detected light intensity. The controlcircuit 98 controls the laser diode 58 so that the maximum lightintensity can be obtained at the peak wavelength.

The output light of the mirror 56 passes through the coupler 62 and thelight isolator 60, and is output to the mirror 28. Part of the outputlight extracted by the coupler 62 is supplied to the photodetector 64,which detects the intensity of the output light. The automatic levelcontrol circuit 24, which functions as the fourth control part, controlsthe laser diode 16 on the basis of the detected light intensity so thatthe intensity of the output light is constant.

That is, the intensity of the exciting lights respectively emitted fromthe laser diodes 16 and 58 are controlled so that the ASE peakwavelength is the same as that obtained in the adjustment by varying theamount of attenuation of the attenuator 92 on the basis of the intensityof the input light. Hence, the wavelength-dependence of the gain of thelight amplifier becomes independent of the intensity of the input light.

In the fourteenth embodiment of the present invention, the output of thefirst-stage light amplifier is amplified by the second-stage lightamplifier having a constant gain, so that theinput-light-intensity-dependence of the wavelength-dependence of thegain can be suppressed and an increased signal gain can be obtained.

FIG. 17 is a block diagram of a light amplifier according to a fifteenthembodiment of the present invention in which the configuration shown inFIG. 2 is modified so that the front stage between the light inputterminal 10 and the light isolator 26 shown in FIG. 2 is replaced by theconfiguration shown in FIG. 10. The input signal light in the 1550 nmband applied to the light input terminal 10 passes through the lightisolator 12 used to prevent oscillation, and is supplied to the mirror104 through the coupler 100 and 102. The signal light with the excitinglight combined is supplied to the erbium-doped fiber 42.

As has been described previously, the mirror 106 is supplied, via thecoupler 110, with the 980 nm exciting light generated by the laser diode16, and combines the signal light with the supplied exciting light. Inthe erbium-doped fiber 42, the signal light is amplified by the 980 nmexciting light. The hole burning or ESA is caused by the 850 nm excitinglight. The signal light amplified by the erbium-doped fiber 42 is outputvia the mirror 106, and passes through a coupler 108 and the lightisolator 60 used to prevent oscillation. Then, the signal light isoutput via the wavelength distribution mirror 28.

Part of the 980 nm exciting light is separated by the coupler 100, andthe photodetector 112 detects the intensity of the exciting light. Partof the input signal light is separated by the coupler 102, and theintensity thereof is detected by the photodetector 114.

Part of the output signal light is separated by the coupler 108, and theintensity thereof is detected by the photodetector 116. The automaticlevel control circuit 118 controls the laser diode 16 so that theintensity of the output signal light is constant. Part of the 980 nmexciting light separated by the coupler 110 is supplied to thephotodetector 120, which detects the intensity of the exciting light.

The loss computation part 122 computes a loss of the 980 nm excitinglight from the intensities of the output lights of photodetectors 120and 112. The gain computation part 124 computes the gain of the signallight from the intensities of the lights detected by the photodetectors116 and 114. The control circuit 126 obtains the ratio of the logarithmof the loss of the exciting light to the logarithm of the gain of thesignal light, and thereby controls the laser diode 58 so that theinverted population coefficient is constant and thewavelength-dependence of the gain is not varied independently of theintensity of the input light. In the above manner, the intensity of theoutput light of the erbium-doped fiber 42 is optimized so that it has anoptimal value with respect to the next-stage light amplifier having theerbium-doped fiber 32.

The mirror 28 combines the signal light with the 980 nm or 1480 nmexciting light supplied from the laser diode 16, the combined lightbeing applied to the erbium-doped fiber 32. The back power dependent onthe intensity of the exciting light from the laser diode 16 is suppliedto the automatic level control circuit 34, which controls the laserdiode 30 so that the intensity of the exciting light emitted from thelaser diode 30 is constant.

The erbium-doped fiber 32 forms a light amplifier in cooperation withthe mirror 28 and the laser diode 30, and has a larger amount of erbiumor a longer length than the erbium-doped fiber 42. The erbium-dopedfiber 32 is supplied with the signal light having the intensityoptimized by the erbium-doped fiber 42 and the exciting light having theoptimized intensity from the laser diode 30. That is, the intensity ofthe input light to the erbium-doped fiber 32 is made constant. Thesignal light amplified by the erbium-doped fiber 32 passes through thelight isolator 36 used to prevent oscillation, and is output to thelight output terminal 38.

In the fifteenth embodiment of the present invention, the output of thefirst-stage light amplifier is amplified by the second-stage lightamplifier having a constant gain, so that theinput-light-intensity-dependence of the wavelength-dependence of thegain can be suppressed and an increased signal gain can be obtained.

The present invention is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the present invention.

What is claimed is:
 1. A light amplifier comprising:an amplifier partfor amplifying input wavelength-multiplexed signal light in accordancewith exciting light supplied thereto: a first photodetector detectingintensity of the input wavelength-multiplexed signal light; a secondphotodetector detecting intensity of an output wavelength-multiplexedsignal light obtained from said amplifier part by amplifying said inputwavelength-multiplexed signal light; and a gain control circuitcontrolling the exciting light based on said intensity of the inputwavelength-multiplexed signal light obtained by the first photodetectorand said intensity of the output wavelength-multiplexed signal lightobtained by the second photodetector.
 2. The light amplifier as claimedin claim 1, further comprising an isolator which allows the inputwavelength-multiplexed signal light to be applied to the amplifier part.3. The light amplifier as claimed in claim 1, further comprising awavelength distribution mirror which combines the inputwavelength-multiplexed signal light and the exciting light.
 4. The lightamplifier as claimed in claim 1, further comprising a first couplerwhich separates the input wavelength-multiplexed signal light input tobe applied to the first photodetector.
 5. The light amplifier as claimedin claim 1, further comprising a second coupler which separates theoutput wavelength-multiplexed signal to be applied to the secondphotodetector.
 6. The light amplifier as claimed in claim 1, wherein theamplifier part comprises an erbium-doped optical fiber amplifier.
 7. Thelight amplifier as claimed in claim 1, further comprising an isolatorthrough which the output wavelength-multiplexed signal from theamplifier part passes and is output from the light amplifier.
 8. Thelight amplifier as claimed in claim 1, wherein the automatic gaincontrol circuit controls the gain of the amplifier part so as to bemaintained at a constant level.
 9. A light amplifying method comprisingthe steps of:amplifying input wavelength-multiplexed signal light inaccordance with exciting light supplied thereto; detecting the inputwavelength-multiplexed signal light; detecting said amplified inputwavelength-multiplexed signal light; and controlling the exciting lightfor a gain of an amplifying operation so as to be maintained at aconstant level, which is determined by an intensity of the inputwavelength-multiplexed signal light and an intensity of the amplifiedinput wavelength-multiplexed signal light.
 10. A light amplifiercomprising:an amplifier part for amplifying input wavelength-multiplexedsignal light in accordance with exciting light supplied thereto, saidamplifier part being an erbium-doped optical fiber amplifier; a bandpassfilter eliminating unwanted components without the inputwavelength-multiplexed signal light amplified by said amplifier part; afirst photodetector detecting intensity of the inputwavelength-multiplexed signal light; a second photodetector detectingintensity of an output wavelength-multiplexed signal light obtained fromsaid amplifier part by amplifying said input wavelength-multiplexedsignal light and filtered by said band pass filter; and a gain controlcircuit controlling the exciting light for a gain of said amplifier partdetermined by said intensity of the input wavelength-multiplexed signallight obtained by the first photodetector and said intensity of theoutput wavelength-multiplexed signal light obtained by the secondphotodetector.
 11. A light amplifier comprising:an amplifier part foramplifying an input wavelength-multiplexed signal light in accordancewith exciting light supplied thereto; a first photodetector detectingintensity of the input wavelength-multiplexed signal light; a secondphotodetector detecting intensity of an output wavelength-multiplexedsignal light obtained from said amplifier part by amplifying the inputwavelength-multiplexed signal light; a control circuit controlling again of said amplifier part determined by said intensity of the inputwavelength-multiplexed signal light obtained by the first photodetectorand said intensity of the output wavelength-multiplexed signal lightobtained by the second photodetector; and a control part which performsa control operation so that an intensity of an amplified light signaloutput from the amplifier part is controlled.